The gain of an amplifier is dependent on the mutual conductance “gm” of each transistor constituting the amplifier and the load connected to the transistor. Mutual conductance gm varies depending on the manufacturing process of the transistor and the temperature of the transistor. Therefore, compensation of mutual conductance gm is important to compensate the gain of the amplifier. Hence, a conventional amplifying circuit compensates gain by using a circuit that compensates mutual conductance gm (hereinafter, “gm compensating circuit”).
FIG. 13 is a circuit diagram of a conventional gm compensating circuit. As depicted in FIG. 13, in the conventional gm compensating circuit, a PMOS transistor 103 and an NMOS transistor 104 are connected in series between a positive power terminal (AVD) 101 and a ground (AVS) 102. A PMOS transistor 105, an NMOS transistor 106, and a resistor 107 are connected in series between the positive power terminal (AVD) 101 and the ground (AVS) 102.
Gate terminals of the PMOS transistors 103 and 105 are commonly connected to a drain terminal of the PMOS transistor 105. Gate terminals of the NMOS transistors 104 and 106 are commonly connected to a drain terminal of the NMOS transistor 104. In this gm compensating circuit, the mutual conductance gm of the NMOS transistor 106 is controlled to be constant. A gm compensating circuit configured as above is disclosed in: for example, Behzad Razavi, “Design of Analog CMOS Integrated Circuits”, McGraw-Hill Publishing Co., USA, Oct. 1, 2003, p. 393.
FIG. 14 is a circuit diagram of a conventional amplifying circuit. As depicted in FIG. 14, the conventional amplifying circuit is configured by a biasing unit including a current source 111 and an NMOS transistor 112 connected between the positive power terminal (AVD) 101 and the ground (AVS) 102, and an amplifying unit including a loading resistor 113 and an NMOS 114 connected between the positive power terminal (AVD) 101 and the ground (AVS) 102. Gate terminals of the NMOS transistors 112 and 114 are commonly connected to a drain terminal of the NMOS transistor 112.
A gate terminal of the NMOS transistor 114 is connected to an input terminal (IN) 116 through a capacitor 115. A drain terminal of the NMOS transistor 114 is connected to an output terminal (OUT) 117. In the conventional amplifying circuit, by using the gm compensating circuit as depicted in FIG. 13 to equalize the gate widths and the gate lengths of the NMOS transistor 106 of the gm compensating circuit and NMOS transistor 114 of the amplifying circuit, the same amount of current that flows through the NMOS transistor 106 of the gm compensating circuit is caused to flow from the current source 111 of the amplifying circuit by a current mirror. Thereby, a gate-source voltage Vgs (gate bias Vg) of the NMOS transistor 114 of the amplifying circuit is controlled and the mutual conductance gm of the NMOS transistor 114 is compensated. Therefore, the gain is compensated.
FIG. 15 is a schematic depicting the relation between the mutual conductance gm and the gate bias Vg of the conventional amplifying circuit. As depicted in FIG. 15, variation of a characteristic curve under low-temperature conditions is steep compared to a characteristic curve under room-temperature conditions. On the other hand, variation of a characteristic curve under high-temperature conditions is gradual. To keep the mutual conductance gm constant when the temperature varies, the gate bias Vg is varied according to the variation of the temperature as depicted by the arrow in FIG. 15.
However, the above conventional amplifying circuit has the following problem. When the amplitude of a signal to be amplified is large and the gate bias point is high, if the amplifying circuit is in the high-temperature condition and the mutual conductance gm is low, the gate bias Vg that is able to keep the mutual conductance gm constant exceeds a controllable range (see FIG. 15). Therefore, the mutual conductance gm may not be compensated by merely controlling the gate bias Vg. In this case, the gain of the amplifying circuit is unable to be compensated.